Traditionally, optical matched filters for pattern recognition applications were produced using two techniques. The first technique employed holographic interferometric architectures similar to the Mach-Zehnder arrangement., The optical Fourier transform, produced by a lens, was interfered with an off-axis plane carrier light wave. This technique has come to be called the Vander Lugt architecture.
In the xf-yf plane, the off-axis plane wave reference beam interferes with the Fourier transform of the input transparency. The phase profile of the reference wave projected onto the flat xf-yf plane is wedge shaped, so that straight fringes are recorded at the location of each spatial frequency component of the in put scene. These fringes behave as microscopic diffraction gratings when illuminated by the Fourier transform of a test scene, and indicate the presence of the original input scene. This type of filter was originally made using photographic emulsion to record the interference pattern. A high-resolution medium was required for recording the fringes that were typically a few wavelengths wide. Once the film was exposed and then developed, it would be placed back in the system. If the input now used to address the filter exactly matched the spatial frequency information in the filter, the reference beam would be recreated from each exposed area. This collection of plane waves was then typically Fourier-transformed by another lens, resulting in the correlation function.
The second technique for producing matched filters involved digitally computing the Fourier transform of the scene of interest, which in general contains both amplitude and phase information, then taking the complex conjugate of the function and displaying the result on an electrically addressable spatial light modulator. The correlation can be directed off-axis if a prescribed phase ramp can be included in the calculation. The most serious problem with this technique is in finding a suitable device for displaying the computed filter. Currently, a device having arbitrary amplitude and phase addressability does not exist. Researchers have been forced to use approximate filters or filters that modulate only phase with an uncontrollable amplitude.
An object of this invention is to perform the complex filter computation optically, and immediately test it--thereby avoiding the time consuming digital calculation and complications involved with photographic emulsions.